Very high contents of met5-enkephalin-like peptides were found in pituitaries of many species if animals wre killed by decaptation. Furthermore, met5-enkephalin immunoreactivity in pituitary was found to consist of high (MW about 1800) and low (MW about 600) molecular weight enkephalin-like peptides. Interestingly, one of the high molecular weight forms was found to be much lower in pituitaries of heroin addicts in comparison to controls. The molecular mechanism underline this heroin induced modification in met5-enkephalin immunoreactivity still remains to be established.